Abstract: Reject waste water adapter coupling fitting, system and installation method for use with a household undercounter reverse osmosis (R/O) water filtration installation associated with an in-counter sink having a garbage disposer with its outlet connected by standard slip fit plumbing fittings to a sink drain trap plumbed to the household sewer system. The R/O reject waste water undercounter drain line is coupled into the sink drain trap in parallel drainage relation with the disposer outlet plumbing fittings by a baffle Tee plumbing fitting having first and second inlets perpendicular to one another with their input drainage liquid flow separated by an internal baffle. The garbage disposer outlet is coupled by a standard slip fit single inlet/outlet conduit plumbing fitting to the baffle Tee first inlet. The baffle Tee has a single outlet slip coupled to the inlet of the trap.